Early attempts at passive intrusion detection systems typically utilized a single infrared sensor to detect radiation emanating from a heat-generating object, typically a human body. The sensor supplies an electrical signal as a result of a change in incident radiation and that signal can be used to trigger an alarm. The primitive infrared intrusion systems suffered from inadequate sensitivity, that is, the change in level of radiation required to reliably trigger the alarm was greater than desired. Furthermore, the single-sensor systems were susceptible to false triggering or false alarm indications and were known to be responsive to irrelevant changes in ambient lighting conditions.
More sophisticated intrusion alarms have used a plurality of infrared sensors and spherical mirrors to collect the infrared radiation and either reflect or refract that radiation upon the individual sensors. See, for example, Mortensen, "Fire and Intrusion's Detection and Alarm Apparatus", U.S. Pat. No. 4,052,716, Oct. 4, 1977. In such systems each sensor can be made more or less sensitive to light emanating from a particular location. (See, Mortensen, supra at columns 3-6 for a thorough explanation of one such system.) By appropriately processing electrical signals derived from the sensors, significant improvements can be made in the performance of the intrusion detector. A particularly helpful technique is to trigger the alarm in response to only differential changes in the sensor signals and to limit the bandwidth of the sensor and/or processing circuitry. Nevertheless, even the more sophisticated systems can be expected to generate spurious alarms if, for example, incident ambient light fails on only one sensor in a dual sensor system. This invention is directed to an intrusion system that discriminates between radiation emanating from stationary and moving objects and triggers an alarm in response to only moving objects.